Support starting DTD and setting workspace roots from integration tests

This is a follow up to https://github.com/flutter/devtools/issues/9176. To emulate the runtime environment where DevTools server was started with a DTD uri passed to it on start, we need to be able to start DTD from integration tests before we start the DevTools server.

This will unblock use cases like testing static DevTools extensions that should be present without a connected app, based on the roots set by the DTD instance that was started by the user's IDE.

To do this, we can re-use this code: https://github.com/dart-lang/sdk/blob/main/pkg/dds/lib/src/devtools/dtd.dart. We may want to expose this so we can import it directly from `devtools_app`.

EDIT: it may be better to run `dart tooling-daemon` directly.